This invention relates generally to surveying of boreholes, and more particularly concerns methods and apparatus which enable significant reductions in well survey time; also it relates to land navigation apparatus and methods.
In the past, the task of position mapping a well or borehole for azimuth in addition to tilt has been excessively complicated, very expensive, and often inaccurate because of the difficulty in accomodating the size and special requirements of the available instrumentation. For example, magnetic compass devices typically require that the drill tubing be fitted with a few tubular sections of non-magnetic material, either initially or when drill bits are changed. The magnetic compass device is inserted within this non-magnetic section and the entire drill stem run into the hole as measurements are made. These non-magnetic sections are much more expensive than standard steel drill stem, and their availability at the drill site must be pre-planned. The devices are very inaccurate where drilling goes through magnetic materials, and are unusable where casing has been installed.
Directional or free gyroscopes are deployed much as the magnetic compass devices and function by attempting to remember a pre-set direction in space as they are run in the hole. Their ability to initially align is limited and difficult, and their capability to remember degrades with time and environmental exposure. Also, their accuracy is reduced as instrument size is reduced, as for example becomes necessary for small well bores. Further, the range of tilt and azimuthal variations over which they can be used is restricted by gimbal freedom which must be limited to prevent gimbal lock and consequent gyro tumbling.
A major advance toward overcoming these problems is described in my U.S. Pat. No. 3,753,296. That invention provides a method and means for overcoming the above complications, problems, and limitations by employing that kind and principal of a gyroscope known as a rate-of-turn gyroscope, or commonly `a rate gyro`, to remotely determine a plane containing the Earth's spin axis (azimuth) while inserted in a bore-hole or well. The rate gyroscope has a rotor defining a spin axis; and means to support the gyroscope for travel in a bore-hole and to rotate about an axis extending in the direction of the hole, the gyroscope characterized as producing an output which varies as a function of azimuth orientation of the gyroscope relative to the Earth's spin axis. Such means typically includes a carrier containing the gyroscope and motor, the carrier being sized for travel in the wall, as for example within the drill tubing. Also, circuitry is operatively connected with the motor and carrier to produce an output signal indicating azimuthal orientation of the rotating gyroscope relative to the carrier, whereby that signal and the gyroscope output may be processed to determine azimuth orientation of the carrier and any other instrument thereon relative to the Earth's spin axis, such instrument for example comprising a well logging device such as a radiometer, inclinometer, etc.
U.S. Pat. No. 4,192,977 improves upon U.S. Pat. No. 3,753,296 in that it provides for use of a "rate gyro" in combination with a free gyroscope, with the rate gyro used to periodically calibrate the free gyroscope. While this combination has certain benefits, it does not provide the unusually advantageous modes of operation and results as are afforded by the present invention. Among these are the enablement of very rapid surveying of boreholes; the lack of need for a free gyroscope to be periodically calibrated; and reduction in time required for surveying slanted boreholes, or particular advantage at depths where high temperatures are encountered.
The improvements in high speed well surveying discussed herein also find application in methods and apparatus for land-vehicle navigation. In present land vehicles, self-contained navigation capabilities have been provided by (1) use of various magnetic-compass direction references and a suitable sensor for measuring distance traveled over the Earth, or (2) use of various free-gyroscope direction references and a suitable sensor for distance traveled over the Earth, or (3) use of complete inertial navigation systems, with or without external position or velocity reference aids. These approaches have generally been of relatively poor accuracy for reasonable costs, or of excessive cost for highly accurate systems. In many land vehicles the large mass of iron-based materials completely prevents the use of magnetic-compass type direction sensors for all but the poorest performancce requirements.
The land-vehicle navigation problem can be seen to have considerable similarity to the high speed well surveying problem. A sensor that provides a measure of vehicle distance traveled over the earth can be seen to provide the same type of information as that provided by the borehole-surveying apparatus wireline that measures the progression distance of the survey tool along the borehole axis. If the land vehicle carries an apparatus to measure inclination and azimuth directions for the vehicle, which apparatus is substantially equivalent to the basic apparatus describe herein for well surveying, then vehicle positioning with respect to the starting point as it travels over the earth surface can be computed. This computation is very similar to that for computing the position of the borehole survey apparatus as it progresses through the earth along the borehole.
In the borehole survey problem the survey tool is confined laterally in the borehole by the borehole dimensions and is generally traveled along the borehole direction at a constant velocity. These constraints reduce tool accelerations to negligible values so that no significant errors are introduced in computing azimuth, inclination or tilt, and position of the tool along the borehole. Also, the borehole survey problem normally applies to paths ranging from vertical to near horizontal.
The land vehicle navigation problem is concerned with substantial horizontal accelerations during vehicle travel over the earth, both in the along path and cross-path directions. Also, the vehicle path is nominally in a horizontal plane with equally-expected up and down movement over typical hilly terrain.
This continuation application of a continuation-in-part application addresses the application of unusually advantageous methods and apparatus developed for high speed well survey to the land vehicle navigation problem, and describes modifications and extensions that permit highly accurate navigation in the presence of the dynamic acceleration environment of the land vehicle traveling over the surface of the earth.